So I have figured out a way to reproduce the THD-- problem. It is definitely a software bug. When the THD measurement upper frequency is set to the Nyquist limit of either 24k or 48k depending on the sampling rate, the problem occurs. While it is not possible to set the upper frequency to the Nyquist limit in the THD screen (gets invalid value error - BTW, I though I saw an update that was to make that possible) it is possible for it to get that value from the following procedure:
Set the sample rate to 192kps. Now set the THD measurement upper frequency to something over 48kHz. Then, set the sample rate to 96kps. The upper measurement frequency gets set to 48kHz, and the THD measurement goes away.
Here is a settings file that reproduces the problem: Or not, can’t upload a settings file! The key point is that the settings file shows the measurement bandwidth set to 24000,